How Training in Heat Can Boost Your Body

A woman puts on sunglasses before running in the heat.

A decade ago, my wife was presented with an awesome opportunity. She would work at a rural medical center on the eastern coast of the Dominican Republic for a month. I would help where I could. That included training her for a marathon. The only catch was the weather – daytime highs were always right around 90. The lows were 80 if we were lucky. And the humidity? Oh, the humidity.

We knew from some basic research that it would take 10-14 days to acclimate to the heat. What we didn’t know was that our stay at sweltering sea level could have similar effects to training at altitude.

Yes, altitude.

Altitude is a mythical word in the running community. Experts tout it as a panacea for the distance runner’s soul, a magical feat of air pressure that (legally!) changes the very composition of your blood. Even those born without the incredible genetics of the Kenyans and Ethiopians who made its effects famous often see major performance boosts once they train in locales between 5,000 and 8,000 above sea level.

Today, towns in that topographical sweet spot comprise some of the hippest training venues in the United States: Boulder, Colorado; Flagstaff, Arizona; Albuquerque, New Mexico; and Mammoth Lakes, California, just to name a few. Elites (and those who dream of hearing that word) flood these towns for the simple fact that the thin air increases the quantity of oxygen-carrying red blood cells. In theory, this turbocharges your cardiovascular engine, leading to performance gains when you race at sea level.

For the majority of the U.S. population, training at altitude is a rare treat. Heat, on the other hand, we have in spades. Research shows a heat-training protocol can increase cardiac output, plasma volume, tolerance to extreme conditions, and even lower your internal thermostat. This is great news for the many road racers gearing up for a summer of racing and younger people putting in the miles for fall cross country. Making heat work for you, however, takes more than laying out poolside.

[Editor’s note: Running or working out in warm weather can be dangerous. Check out our tips for running safely in hot weather. ]

How Does Heat Affect Your Body?

Two women stand together after running in the heat.

If your grandma used to warn you about the risks of running around in the middle of the summer, she wasn’t wrong.

Extreme temperatures increase the stress on your body many-fold. Blood that might normally be used to power your muscles is redirected to your skin to make sweat that’ll help you thermoregulate. As your sweat rate increases, so does the risk for significant dehydration—cognitive performance can decline after a 2 percent drop in bodyweight.

Your heart rate rises accordingly, trying to move blood to the many places it’s needed. Humidity amplifies this effect, reducing the cooling effect of sweat as it pools on your skin instead of evaporating. You feel thirsty, strained and hot. The end result is that a given pace is more difficult to maintain. And if you’re racing, good luck: the faster the pace, the higher the magnitude of these effects.

The body, however, is clever. Expose it to a given stimulus enough and it will adapt out of necessity. Joshua Guy and his colleagues published an article in the journal Sports Medicine (March 2015) looking at this very topic and found that after 8 to 14 days of heat exposure athletes exercised with lower heart rates than before at the same speed, sweated earlier and had lower core temperatures. These adaptations to heat training made the athletes more efficient in all conditions.

This seemed to confirm the findings of Santiago Lorenzo’s groundbreaking 2010 study. The then-University of Oregon professor found that after a 10-day exercise protocol in the heat (100 degrees), the heat-trained athletes increased their VO2 max and power output by 8 percent and 5 percent, respectively, in hot conditions. The bigger surprise was their VO2 max, time trial performance and power output at lactate threshold were also all 5-6 percent higher than before. Plasma volume and maximal cardiac output were also up 6-9 percent for the heat trained group. The control group training in 55-degree weather showed no significant gains in any of these categories.

How to Implement Heat Training

A man and two women run on a trail in the heat.

It’s easy to see journal articles touting the efficacy of heat training and want to jump in immediately. Before we do, I want to point out several things from these articles:

The cyclists in Lorenzo’s study were well-trained athletes exercising in a controlled environment. Though the temperature was very high in the heat chamber, the humidity was quite low (30 percent).

There were only 20 athletes in Santiago’s study. Other studies show similar constraints.

Guy’s paper was a review paper, and more research needs to be done to see if these gains translate directly to runners in a sustainable manner and how long they last.

There are also tradeoffs inherent in heat training. No matter how well you adapt, you’ll always run a 5K slower in 90-degree heat versus 50-degree weather. The same goes for intervals or tempos. While sprinters may find running in warm conditions advantageous, distance runners suffer sharp declines as the duration increases.

For those reasons, I recommend the following as the best way to implement heat training into your own program:

Start with only easy runs in the heat. To see how well you’ll tolerate extreme summer training, run shorter, easier runs during the heat of day, and save workouts for the cooler mornings or less-humid evenings.

When you implement workouts, start with shorter intervals. One of the reasons we find ourselves wanting to stop or slow down during summer training is because it’s easier to cool the body when it’s not generating additional heat. By running sprints and shorter intervals with longer recoveries, you’ll give yourself some literal chill time to assess whether you’re acclimating properly to the stress of heat and humidity.

Run in areas that have water fountains or carry a handheld water bottle. If you have the option, bring the electrolytes! Dehydration is a guarantee during the hotter months, but total water lost is less critical than intracellular hydration, which can be maintained by ingesting small to moderate amounts of fluids during exercise. (This is most evident in big city marathons, where elites may lose 3-6 percent of their bodyweight from dehydration but not slow down). If you have the option to mix electrolytes with your water, all the better. Salt, magnesium, and potassium help get water into your cells and replace what’s lost during sweating.

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I can’t say definitively where spring went. I’m pretty certain we went from an endless winter to an extreme summer in the blink of an eye. But the silver lining is there. Use caution and discretion implementing it and you, too, may find yourself running the best times of your life this fall after a much more enjoyable June, July and August.

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